If vs boolean assignment speed C++

I have a function that is run ~60 times per second on a Nintendo DS (~50Hz CPU, ~3MB RAM). In this function I need to execute changed() only if the boolean has changed from false to true. Does the if statement increase the speed or decrease it?

bool variable = false; //Externally changed and accessed variable
bool oldVar; //Previous state of variable

void update()
{
    //////////////////////////////////
    if (!variable)
    //////////////////////////////////
        oldVar = variable;

    if (variable && !oldVar)
    {
        oldVar = true;
        changed();
    }
}

Or is there a better way to do this entirely?